A variety of track access systems have been proposed in connection with conventional optical disk driving apparatuses. The following is one of those track access systems, i.e., so called a track counting system. The track counting system is arranged such that moving means such as a linear motor causes an optical head to move to a target track from the current position which is detected according to the number of the pulses of a traverse signal, the traverse signal being detected by the optical head and varying depending on the number of the tracks that the optical head actually crossed.
The following describes the conventional track counting system. As shown in FIG. 4(a), the surface of an optical disk 60 is provided with guide grooves 61 having a predetermined distance therebetween with respect to a radial direction of the optical disk 60. Each track 62 is defined between two adjacent guide grooves 61.
A light beam 63 moves with respect to the radial direction of the optical disk 60 while crossing the tracks 62 along, for example, an arrow 64. It is assumed that the light beam 63 moves from the inner part of the optical disk 60 toward the outer part thereof during the zone defined by A and B as shown in FIG. 4(a) when track access is performed. It is also assumed that the light beam 63 moves from the outer part of the optical disk 60 toward the inner part thereof during the zone defined by B and C as shown in FIG. 4(a) when the track access is performed. Further, the light beam 63 actually moves in a direction perpendicular to the track 62 or substantially perpendicular to the track 62 during the track access. However, the light beam 63 on the optical disk 60 traces such that the light beam 63 crosses in a direction diagonal to the track 62, since the optical disk 60 ordinarily rotates even when the track access is performed.
A tracking error signal 65 changes as shown in FIG. 4(b) when the light beam 63 moves along the arrow 64. A total signal 66 changes as shown in FIG. 4(c) when the light beam 63 moves along the arrow 64. The tracking error signal 65 becomes zero in the center of the track 62 in a width direction thereof. The total signal 66 becomes a maximum in the center of the track 62 in the width direction thereof.
In addition, the tracking error signal 65 is given by a difference signal between the outputs from the respective photoreceiving parts of a divided photodetector (not shown), for example. The total signal 66 is given by a sum signal between the outputs from the respective photoreceiving parts of the divided photodetector.
A binary tracking error signal 67 is shown in FIG. 4(d), the binary tracking error signal 67 being given by making the tracking error signal 65 in a binary logic condition. A land/groove discrimination signal 69 is shown in FIG. 4(e), the land/groove discrimination signal 69 being given by comparison of the total signal 66 to a slice level 68 (see FIG. 4(c)) by means of a comparator (not shown) making the compared result in the binary logic condition. Each guide groove 61 (groove) corresponds to a low level of the land/groove discrimination signal 69. Each track 62 (land) corresponds to a high level of the land/groove discrimination signal 69.
A direction signal 70 is shown in FIG. 4(f), the direction signal 70 being given by latching a level of the land/groove discrimination signal 69 in response to a rising edge of the binary tracking error signal 67. The direction signal 70 becomes a low level when the light beam 63 moves from the inner part of the optical disk 60 toward the outer part thereof. The direction signal 70 becomes a high level when the light beam 63 moves from the outer part of the optical disk 60 toward the inner part thereof.
An edge detection signal 71 is shown in FIG. 4(g), the edge detection signal 71 containing pulses, each of which is outputted for a predetermined time from a rising edge of the binary tracking error signal 67. The edge detection signal 71 corresponds to a timing that the light beam 63 crosses the guide groove 61 when the light beam 63 moves from the inner part of the optical disk 60 toward the outer part thereof. The edge detection signal 71 corresponds to a timing that the light beam 63 crosses the track 62 when the light beam 63 moves from the outer part of the optical disk 60 toward the inner part thereof.
A up-signal 72 is shown in FIG. 4(h) and a down-signal 73 is shown in FIG. 4(i). The respective signals 72 and 73 are given by the selection of the edge detection signal 71 according to a logic level, i.e., high level or low level of the binary logic condition. More specifically, the up-signal 72 is generated in response to the edge detection signal 71 when the direction signal 70 is a low level. The down-signal 73 is generated in response to the edge detection signal 71 when the direction signal 70 is a high level. The number of the pulses of the up-signal 72 corresponds to the number of the tracks 62 which the light beam 63 crossed when it moves from the inner part of the optical disk 60 toward the outer part thereof. The number of the pulses of the down-signal 73 corresponds to the number of the tracks 62 which the light beam 63 crossed when it moves from the outer part of the optical disk 60 toward the inner part thereof.
Accordingly, the amount of movement of the optical head with respect to the radial direction of the optical disk 60 can be detected when a up/down counter (not shown) counts the up-signal 72 and the down-signal 73.
However, there are some cases where an accurate amount of movement of the optical head with respect to the radial direction can not detected in the above-mentioned conventional optical disk driving apparatus.
The following is an example of such cases. The total signal is effected when an optical disk having different reflectance is loaded into the apparatus. This causes a DC component and an AC component of the total signal to change. A total signal of an optical disk has the characteristic that the ratio of a DC level with respect to an AC component amplitude is relatively great. Therefore, when the total signal is compared to a predetermined slice level even though the total signal is effected by the reflectance change of the loaded optical disk, the amount of movement of the optical head can not detected with accuracy in this case.
Some restrictions can be given by adopting a standard and keeping thereof with regard to the problem of the different reflectance. However, the tolerance is too wide for the accurate discrimination. This causes the apparatus to misjudge the direction in which the optical head moves and causes the up/down counter to count in a wrong manner. This results in a problem that the detection of the position where the optical head is located can not be performed accurately.
In order to overcome the above-mentioned difficulties, the following method is proposed: when an optical disk is loaded into the apparatus, a total signal is sampled to have the average thereof and is adopted as a slice level for the comparison. However, this proposed method causes the apparatus to have a large scale circuit configuration. Moreover, another slice level must be set depending on the position of the optical head when a disk having a large distribution of the reflectance is loaded into the apparatus. This is not a practical way of overcoming the above-mentioned difficulties.